QCA7e_ch07_szb_handouts

QCA7e_ch07_szb_handouts - Daniel C. Harris Quantitative...

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1 Quantitative Chemical Analysis Seventh Edition Quantitative Chemical Analysis Seventh Edition Chapter 7 Let the Titrations Begin Daniel C. Harris Daniel C. Harris Chapter Highlights •T i t ra t ions – Stochiometry, calculations – Spectrophotometric titrations – Precipitation titrations • Titration curve • Titration of a mixture • End-point detection – Vollhard – Fajans – Efficiency in experimental design
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2 The Evolution of the Buret Titrations • In a titration, increments of reagent solution – the titrant – are added to analyte until their reaction is complete. • From the quantity of titrant required, one can calculate the quantity of analyte that was present. Titrant is usually delivered from a buret.
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3 When Does It Work? • Principal requirements for a titration reaction: – Large equilibrium constant – Fast reaction rate • That is, each increment of titrant should be completely and quickly consumed by analyte until the analyte is used up. • The most common titrations: – acid-base, oxidation-reduction, complex formation, precipitation reactions
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4 Equivalence Point •T h e equivalence point occurs when the quantity of added titrant is the exact amount necessary for stoichiometric reaction with the analyte. • Example: 5 mol of oxalic acid react with 2 mol of permanganate in hot acidic solution: • If the unknown contains 5.000 mmol of oxalic acid, the equivalence point is reached when 2.000 mmol of have been added. End Point • Equivalence point – ideal (theoretical) result we seek in a titration • What we measure is the end point – marked by a sudden change in a physical property of the solution • In the example: – the abrupt appearance of the purple color of permanganate in the flask
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5 Methods to Detect the End Point • Detecting a sudden change in the voltage or current between a pair of electrodes •Obse rv ing an indicator color change – An indicator is a compound with a physical property (usually color) that changes abruptly near the equivalence point. – The change is caused by the disappearance of analyte or the appearance of excess titrant. • Monitoring absorption of light Titration Error •The difference between the end point and the equivalence point is an inescapable titration error . • By choosing a physical property whose change is easily observed, the end point can be very close to the equivalence point. • Estimate the titration error with a blank titration: – Titrate a solution containing no analyte. – See how much is needed to produce observable change in the solution. – Subtract this volume.
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6 Primary Standard • The validity of the result depends on knowing the amount of one of the reactants used. • If a titrant is prepared by dissolving a weighed amount of pure reagent in a known volume of solution, its concentration can be calculated.
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QCA7e_ch07_szb_handouts - Daniel C. Harris Quantitative...

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